At the present time powder metallurgy techniques are utilized for the fabrication of engineered porous metal structures. Typically, such porous metal components are designed having particular properties and for specific applications. The utility of the components extends to battery materials, friction parts, electronic and electrical components, and the like. However, with expanding markets, applications for specialty porous metal products are constantly increasing and diversifying.
Broadly, powder metallurgy processes involve powder synthesis, compaction and then sintering of the structural constituents. Such processes encompass powder sintering, slip forming, slip casting and fibre metallurgy. (Porous Metals. V. Shapovalov MRS Bulletin April 1994).
Powder sintering consists of introducing the metal powder starting material into a die, compacting it, and partially sintering the compressed powder at a selected temperature to attain the desired porosity. This process has inherent disadvantages. Controlled sintering at low temperatures and/or the use of a coarse powder particle size are detrimental to the mechanical strength of the formed component. Furthermore, pore size is limited by particle size, large pores requiring the use of large particles which do not sinter well. Also, the density of pressed powder parts can be non-uniform. Deleteriously, too, using conventional powder metallurgy, it is difficult to produce thin-walled structures because of the weakness of the green body. Also, producing a porous metal product exhibiting controlled pore geometry is usually not feasible.
Exemplary of conventional prior art is the disclosure of U.S. Pat. No. 3,311,505 of a gas electrode, adapted for use in a fuel cell, which electrode is made up of a sintered metal substrate having a formed surface deposit of carbon thereon. The substrate exhibits a porous volume representing forty to eighty percent of its apparent volume.
Porous nickel plates useful for electrochemical devices may be produced by the process described in U.S. Pat. No. 3,796,565 issued to Hancock et al. Additionally, in U.S. Pat. No. 3,799,808 to H. A. Hancock, there is disclosed an elongated, self-supporting porous nickel plate. The process for producing the porous nickel plate comprises applying a layer of slurry containing nickel powder, a volatile liquid and a binder to a thermally decomposable carrier film. The volatile fraction is evaporated to provide a dry layer of nickel powder in admixture with the binder. The dry layer is pressed into a network of reinforcing lines and sintered to thereby volatilize the binder and carrier. The disadvantages of these nickel plates resides, fundamentally, in their lack of mechanical strength especially in the green body state.
Diverging somewhat, processes for the fabrication of ceramic components are various and well-documented in the literature. Amongst such processes is the tape casting process which is primarily known for the manufacture of ceramics used in electronic applications as described by Mistler, R. E. et al. (1978) Tape Casting of Ceramics, in: Ceramic Processing Before Firing G. Y. Onoda and L. L. Hench, eds., Wiley-Interscience, 411-448.
Tape casting techniques involve, in general, preparing a colloidal suspension comprising a ceramic powder, a binder system, a plasticizer and a solvent. The suspension is cast into a thin sheet, and air dried yielding a green body. The tape is subjected to a burnout-cycle to remove pyrolysable slurry additives forming a friable brown body which is subsequently sintered to yield the final product.
With the changing demands for porous metal products, there has arisen a need for more precisely engineered components, which as outlined above, the existing powder metallurgy processes do not fully meet.